An adsorbent that adsorbs and desorbs water has various uses. For example, a dehumidifier can utilize an adsorbent that adsorbs water at low temperature, while desorbs water when it is heated at high temperature. Moreover, a heater may humidify like a humidifier by desorbing adsorbed water, at high temperature, which is adsorbed from outside environment at low temperature, if an adsorbent is used in a heater. Likewise, a cooler, if an adsorbent is incorporated, may decrease the humidity of a room by adsorbing water at low temperature. The adsorbed water may be desorbed to outside by heating at high temperature. Air conditioners and humidity controllers (humidistats) that utilized these concepts are suggested in U.S. Pat. Nos. 6,978,635, 6,959,875 and 6,675,601. However, there is no detailed explanation on the adsorbents used in the equipments. It is said that silica gels, zeolites or ion-exchangers are used in the equipments or it is just stated that adsorbents are used for the equipments. Moreover, the adsorbents have drawbacks of low adsorption capacity, and the desorption temperature is higher than 100° C., leading to the increase of operation costs.
Therefore, it is highly desirable to develop an adsorbent that desorbs water even at low temperature and the difference between adsorption capacity and the remaining water after desorption is large. However, there always remain problems of difficult desorption if the adsorption capacity is high; and the difference between adsorption capacity and the remaining water after desorption is small when the adsorption capacity is small. The present invention can be completed by confirming that, by utilizing a hybrid inorganic-organic material having very large surface area, the adsorption capacity is large and most of the adsorbed water can be desorbed at the temperature of 100° C. or below.
In the present invention, the hybrid inorganic-organic material, which is used as a water adsorbent, can be defined as a porous coordination compound formed by central metal ions and organic ligands that coordinate to the metal ions. The hybrid inorganic-organic material contains both organic species and inorganic species in the framework structure and means a crystalline compound having pore structure of molecular size or nano size. Hybrid inorganic-organic material is a wide terminology and generally called as porous coordination polymer (Angew. Chem. Intl. Ed., 43, 2334. 2004) or metal-organic frameworks (Chem. Soc. Rev., 32, 276, 2003). Recently, the study on these materials has been newly developed by the combination between molecular coordination bonds and materials science, and these materials have been actively investigated in these days because these materials, due to high surface area and pore of molecular dimensions or nanometers, can be used as versatile nanomaterials.
These materials can be synthesized by various methods. Typical synthesis methods include solvent diffusion at room temperature, hydrothermal synthesis at high temperature in the presence of water solvent and solvothermal synthesis using an organic solvent (Microporous Mesoporous Mater., 73, 15, 2004; Accounts of Chemical Research, 38, 217, 2005).
The synthesis of a hybrid inorganic-organic material can be executed, similar to the synthesis of inorganic porous materials including zeolites and mesoporous materials, under autogenous pressure in the presence of water or suitable solvents at temperature higher than the boiling point of the solvent used in the synthesis. In other words, the hybrid inorganic-organic material have been synthesized, similar to zeolites and mesoporous materials, during several days at high temperature after loading reactants in a high pressure reactor such as an autoclave. The heat source for high temperature has generally been conventional electric heating. For example, an autoclave containing precursors including metal salts, organic ligands and water or solvent was heated using an electric heater or an electric oven at a constant temperature. However, the methods of the previous embodiments using electric heating are not economical because of the long reaction time. Moreover, the methods do not provide hybrid inorganic-organic material with size less than 500 nm, or more preferably less than 100 nm, because amorphous hybrid inorganic-organic materials are obtained when the reaction duration is decreased in order to obtain small crystal particles.
Therefore, in the present invention, hybrid inorganic-organic materials are obtained by using microwave as a heat source in order to overcome the above mentioned problems. As small particles are obtained by microwave heating, the small particles, if used as an adsorbent, have the advantage of easy diffusion. Moreover, the small particles are easy to make a thin film, thick film or membrane. On the other hand, the synthesis by using microwave heating leads to improved performance of water adsorbent because the hydrophobicity and hydrophilicity of adsorbents are improved by the method. Therefore, hybrid inorganic-organic materials can be obtained by using microwave heating under the autogenous pressure after sealing the precursors containing metal salts, organic ligands and solvent such as water or organic solvent. Moreover, the hybrid inorganic-organic materials can be synthesized continuously by the microwave heating.